The present invention relates to the heat treating of workpieces and, in particular, to inductive heating of the workpieces followed by arrested cooling of the workpieces to achieve improved metallurgical properties.
The invention will be described with reference to the austempering of annular configured parts, such as gears; however, the invention, in its broader aspects may be employed for other arrested cooling processes such as martempering wherein improved metallurgical properties are desired.
Austempering has been recognized as an effective heat treating process for increasing the hardness, ductility, toughness and fatigue resistance of certain cast ferrous parts, as well as reducing heat treating distortion, dimensional variation and quench cracking. Parts such as ring gears, pinions and the like, particularly in automotive applications, benefit from such process. In a typical process, the entire part is heated to an austenitizing temperature for an extended soaking period, up to three hours or more, sufficient to provide the desired metallurgical dissolution of the graphite and carbon constituents. Thereafter, the part is immersed in a salt bath, maintained at an appropriate temperature at or above the starting martensite transformation temperature and below the pearlite transformation temperature. The bath media rapidly quenches the part from the austenitizing temperature past the nose of the TTT curve and maintains an elevated uniform part temperature. At this isothermal transformation temperature the formation of lower bainite is promoted to the exclusion of martensite and intermediate transformation products, such as pearlite and upper bainite. After an extended holding period at the isothermal transformation temperature, also oftentimes three hours or more, the part is air or liquid cooled to ambient temperature. In addition to extremely long cycle times, the criticality of the quenching rates has limited the process to small section parts of irons and steels with good hardenability. For low hardenability parts and larger part sections, the isothermal transformation temperature is not uniformly obtained prior to passing through the other elevated transformation regions and consequently inhibiting a uniform bainite transformation.